1. Field of the Invention
The present invention relates to a lenticular lens sheet suitable for projecting an image by using an imaging light source having cell structure such as an LCD (liquid crystal display) or DMD (digital micro-mirror device), and to a process for producing such a lenticular lens sheet.
2. Related Art
Heretofore, there has been known a rear-projection-type television comprising an imaging light source composed of three CRTs (cathode ray tubes) of red, green and blue, and a rear projection screen on which an image is projected by the imaging light source.
In such a projection television, a screen which is a combination of a Fresnel lens sheet and a lenticular lens sheet is used as the rear projection screen. Of these lens sheets, the lenticular lens sheet is required to diffuse light widely, and to be less affected by extraneous light.
FIG. 13 is a view showing one example of a conventional lenticular lens sheet. In a lenticular lens sheet 40 shown in FIG. 13, a lens area 42 containing a plurality of lenticular lenses for condensing light (hereinafter sometimes referred to simply as "lenses") is formed on the light-entering surface 41 of the lenticular lens sheet 40; and a light-emerging surface 44 is formed in the vicinity of the focal point (condensing point) of each lens in the lens area 42. A non-light-emerging area 47 containing light-shielding parts (black stripes) 48 is provided on the light-emerging surface 44, between the focal points of each two lenses in the lens area 42, so that it is possible to diffuse light and to reduce the effect of extraneous light.
In the field of the above-described projection television, there has also been developed a television using an LCD, DMD or the like as the imaging light source. Also in such a projection television, a lenticular lens sheet with black stripes as shown in FIG. 13 is used from the viewpoints of improvement in the light-diffusing property and prevention of the reflection of extraneous light.
However, in the projection television using an LCD, DMD or the like as the imaging light source, a grating pattern originating from the cell structure of the LCD, DMD or the like is projected on the rear projection screen. The lenticular lens sheet for use in the rear projection screen has a cyclic structure with a constant pitch. Therefore, when an image is projected on such a lenticular lens sheet, Moire fringes formed due to the sampling effect of the lenticular lenses may be observed.
In order to prevent the formation of such Moire fringes, it is preferable to make the lenticular lens pitch not greater than 1/3.5 of the grating space of the grating pattern projected. Further, in the projection television using an LCD, DMD or the like, glaring of the projected image called scintillation occurs. To make the lenticular lens pitch small is also useful for reducing this scintillation. It has already been known that, in the lenticular lens sheet with black stripes as shown in FIG. 13, it is generally required to make the distance between the lenticular lenses formed on the light-entering surface and the light-emerging surface not more than approximately 1.3 times the lenticular lens pitch if it is desired to diffuse light widely at a diffusion angle of 40 degrees or more, and to form black stripes on the light-emerging surface.
For this reason, in the lenticular lens sheet with black stripes as shown in FIG. 13, the lenticular lens pitch is made 0.4 mm or less, and the thickness of the lenticular lens sheet, which corresponds to the distance between the lenticular lenses and the light-emerging surface, is made 0.54 mm or less so that Moire fringes formed due to the grating pattern projected on the rear projection screen and the cyclic structure of the lenticular lenses will be vague.
However, in the lenticular lens sheet with black stripes as shown in FIG. 13, when the thickness of the lenticular lens sheet is made small, the rigidity of the lens sheet is decreased, so that it becomes difficult to maintain the lenticular lens sheet flat. Moreover, it is extremely difficult to accurately mold such a thin lenticular lens sheet by means of extrusion molding or the like.
On the other hand, in the above-described projection television using an LCD, DMD or the like, an emergent-side single lenticular lens sheet, an incident-side single lenticular lens sheet or the like which has been colored is also used in order to improve the light-diffusing property and to prevent the reflection of extraneous light.
In the emergent-side single lenticular lens sheet, the shape of a part of a circle or ellipse, or a shape utilizing total reflection is adopted as the shape of the cross section of the lenticular lens. However, in the case where the shape of the cross section of the lenticular lens is a part of a circle or ellipse, total reflection occurs when the lens angle formed with incident light at the base of the lens exceeds the critical angle. Therefore, the viewing angle cannot be made wide. Further, in the case of the shape utilizing total reflection, it is impossible to accurately transfer the lens pattern by means of extrusion molding because of its peculiar shape. Consequently, it is inevitable to produce such a lenticular lens sheet by a cast molding method which is poor in productivity.
FIG. 14 is a view for illustrating the relationship between the inclination of a lens at the point at which light enters into an incident-side single lenticular lens sheet, and the emergent angle of this light. As shown in FIG. 14, in an incident-side single lenticular lens sheet 60, a lens area 62 containing a plurality of lenticular lenses for condensing light is formed on the light-entering surface 61 of the lenticular lens sheet 60. In FIG. 14, the symbol .phi. represents the lens angle (inclination) at the base of each lens in the lens area 62; the symbol .theta. represents the emergent angle of light which has entered into the base of each lens in the lens area 62; the symbol h represents the height of the lens area 62; and the symbol L represents the distance between the incident point (the base of each lens in the lens area 62) and the condensing point.
The relationship between the lens angle .phi. at the base of the lens, and the emergent angle .theta. and the position of the condensing point is shown in the following Table 1, where the refractive index n of the lenticular lens sheet is 1.5, and the lens pitch p is 1.0 mm.
TABLE 1 ______________________________________ (n = 1.5, p = 1.0 mm) .phi.[deg] .theta.[deg] L[mm] H[mm] ______________________________________ 30 15.9 2.69 0.14 40 22.3 1.92 0.19 50 29.7 1.42 0.26 60 38.9 1.08 0.33 70 51.0 0.83 0.42 ______________________________________
It can be understood from the data shown in the above Table 1 hat, in the incident-side single lenticular lens sheet 60 as shown in FIG. 14, it is necessary to make the lens angle .phi. at the base of the lens sixty degrees or more in order to obtain a wide viewing angle with an emergent angle .theta. of 40 degrees or more.
However, in such an incident-side single lenticular lens sheet 60, when the lens angle .phi. at the base of the lens is made large, extraneous light D which has entered from the light-emerging surface 64 side is totally reflected at the lens, and emerges again from the light-emerging surface 64 as shown in FIG. 3B, and this light is observed (see symbols D1, D2, D3 and D4). For this reason, the image contrast is drastically decreased.
It is noted that, in the case of a conventional lenticular lens sheet with black stripes, the light-emerging surface is formed in the vicinity of the focal point (condensing point) of each lens in the lens area and that the distance between the lens area formed on the light-entering surface and the light-emerging surface is equal to "h+L". In the above Table 1, the distance "h+L" is 1.41 when the lens angle .phi. is 60 degrees, and 1.25 when .phi. is 70 degrees. It can thus be understood that it is necessary to make the distance between the lens area formed on the tight-entering surface and the light-emerging surface approximately 1.3 times the lens pitch. For this reason, in the case of the conventional lenticular lens sheet with black stripes, when the lens pitch is made small, the lenticular lens sheet becomes thin, so that such problems that the rigidity of the lens sheet is decreased and that it becomes difficult to mold such a lens sheet are brought about as contrasted in the case of the incident-side single lenticular lens sheet in which it is not always necessary to make the distance between the lens area and the light-emerging surface "h+L".